In Patent Document 1, there is disclosed a brow-by gas treatment device for an internal combustion engine, the device comprising a fresh air introduction passage for introducing a fresh air into a crankcase from the upstream side (with respect to a throttle valve) of an intake air passage and a blow-by gas passage for emitting blow-by gas from the crankcase into the downstream side (with respect to the throttle valve) of the intake air passage. One end of the fresh air introduction passage is connected to between the throttle valve of the intake air passage and an air flow meter disposed upstream therefrom.
In the thus constructed blow-by gas treatment device, a fresh air is introduced into the crankcase by a difference in pressure between upstream and downstream from the throttle valve while a blow-by gas is introduced toward the downstream side of the throttle valve together with this fresh air, and finally a combustion treatment is initiated in a combustion chamber. In a high-speed high-load region where a difference in pressure between upstream and downstream from the throttle valve is small and the generated amount of blow-by gas is large, the blow-by gas flows backwardly through the fresh air introduction passage to be introduced to the upstream side of the throttle valve.
With consideration given to the backflow of blow-by gas in the fresh air introduction passage, Patent Document 2 discloses an arrangement where an oil separator for the fresh air introduction passage and an oil separator for the blow-by gas passage are provided inside a cylinder head cover of an internal combustion engine and the tip end of the fresh air introduction passage and that of the blow-by gas passage are respectively connected thereto.
In such an arrangement where the fresh air introduction passage is connected to between the throttle vale of the intake air passage and the air flow meter, a periodical change in pressure within the crankcase or a pressure pulsation of blow-by gas is apt to propagate to the intake air passage side through the fresh air introduction passage, which makes the air flow meter cause an error in measuring the amount of intake air. Particularly in the case of using a highly responsive air flow meter, an influence of pulsation of blow-by gas is outstandingly exhibited in a low load region (for example, in an idle state) where the amount of intake air is small.